In state of the art receivers, the frequency band of the converted signal corresponds to the useful channel. Accordingly, the radiofrequency signal received on a reception antenna is filtered, in general after transposition to an intermediate frequency or to baseband, by means of a filter disposed in the reception chain upstream of the analog/digital converter. In this way, the latter converts only the energy of the signal in the useful channel. When the receiver is a multichannel receiver, channel selection means, comprising a selective filter, make it possible to select the useful channel from among a plurality of different channels. In a manner known per se, a variable-gain amplifier can precede the analog/digital converter in the reception chain, so as to tailor the power level in the useful channel to the input dynamic swing in power of the converter. This is achieved by virtue of appropriate automatic gain control means.
Attempts are currently being made to dispose the analog/digital converter nearer to the reception antenna, and in particular upstream of the channel selection means. Hence, the signal converted by the analog/digital converter has a determined frequency band containing at least one useful channel, in general from among several different channels. The width of the frequency band of the converted signal, or converted band, is then greater than that of the useful channel. This converted band is determined by the (analog) filters disposed upstream of the converter. The channel selection means, disposed downstream of the analog/digital converter in the reception channel, then comprise a mixer and one or more digital filters for selecting the useful channel, before digital demodulation and decoding.
However, the power of the radiofrequency signal received on the reception antenna varies over time. These variations may be due to the appearance or the disappearance of obstacles between the transmitter and the receiver, to the appearance or the disappearance of other signals in the frequency band occupied by the signal, or to “fading” when there is a relative motion of the receiver with respect to one of the transmitters. One speaks of propagation of dynamic type in a channel when there is a relative motion of the receiver with respect to the corresponding transmitter, and of propagation of static type in the converse case. Fading is considerable when the Doppler frequency of f0×v/c is considerable, where fo is the central frequency of the channel, v is the relative speed of the receiver with respect to the transmitter and c is the speed of light. It is noted that when a signal is situated in a “fading hole” its power may become very small. The decrease in power of the signal in a “fading hole” is of short duration. In fact, the shorter the duration of the “fading hole”, the smaller the power of the signal in the “fading hole”.
Moreover, the input dynamic swing in power of the analog/digital converter is limited above by an upper limit value beyond which the converter is saturated, and below by a lower limit value beneath which the signal can no longer be distinguished from the noise introduced by the converter. One speaks of saturation value to designate said upper limit value, and of noise floor to designate said lower limit value. By convention, the desired power levels and the mean power values indicated subsequently in this document may be expressed in decibels (dB) with respect to the noise floor of the converter.
Furthermore, any radiocommunication system complies with specifications which determine the sensitivity and the rejection of the system with regard to the useful channel, as a function of the type of propagation in this channel. The sensitivity of the system corresponds to the minimum power level of the signal in the useful channel (signal-to-noise ratio), at which the system must still operate. The rejection of the system corresponds to the maximum power level which must be tolerated by the system inside the converted band, in the channels neighboring the useful channel. The sensitivity in the static case is less than the sensitivity in the dynamic case, and the rejection in the static case is greater than the rejection in the dynamic case. In one example, the dynamic sensitivity is equal to 15 dB above the noise floor of the converter, and the dynamic rejection is equal to 45 dB. Moreover, the static sensitivity is equal to 7 dB above the noise floor of the converter and the static rejection is equal to 70 dB. The input dynamic swing in power of the converter which is necessary is therefore equal to 60 dB in the dynamic case, to 77 dB in the static case, and hence to 85 dB to cover both the static case and the dynamic case if the useful signal level is fixed at the same level in both cases. At a rate of 6 dB per digit, a converter operating on 15 bits at output is therefore necessary.